Flame retardant yarn

ABSTRACT

The flame retardant yarn of the present invention is made by blending flame retardant viscose fiber and at least one high tenacity flame retarding fiber, the at least one high tenacity flame retarding fiber is flame retardant polyester or flame retardant finishing viscose fiber, wherein the flame retardant viscose fiber is 40˜90% by weight of the flame retardant yarn. The flame retardant yarn takes advantage of the superior flame retardance of the flame retardant viscose fiber, and takes the flame retardant viscose fiber to blend with other good tenacity and spinnability fiber by adjusting amount of raw materials in reasonable balance and avoiding disadvantages of the raw materials to optimize advantages of the flame retardant viscose fiber such as flame retardancy and tenacity so as to provide the flame retardant viscose fiber with high flame retardancy, long lasting flame retardancy effect, strength and elongation performance and good abrasion resistance. Fabric made by the flame retardant yarn not only has good flame retardancy, but also has good elasticity due to high tenacity of the flame retardant yarn.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally relates to a technical field of the textile, and more particularly to a flame retardant yarn.

2. Related Art

Strong flame retarding fabrics are mainly applied to home textiles such as mattress, pillowcases, etc. The flame retarding fabrics, on one hand, meet the requirements for flame retardancy standard such as 16CFR1633 U.S. standard for mattress flame retardancy. On another hand, the fabrics have higher fiber strength as well. Conventionally, the flame retarding fabrics are made by blends of glass fiber and viscose fiber and being soaked with flame retardant. However, this technique has defeats. The glass fibers have properties of high thermal resistance, non-flammable form and corrosion resistance but they may cause allergic reaction and even contact dermatitis. The IARC (International Agency for Research on Cancer) of the W.H.O. (World Health Organization) had added the glass fibers into the list of three carcinogen types. Further, the fibers in initial period indeed produce good flame retardancy because they are made from fiber filament and soaked in liquid with flame retardant. However, it does not stand washing at all. With the increase in time of use and through several washing times, the flame-retarding effect must be broken down until it has disappeared.

U.S. Pat. No. 5,506,043 discloses a high temperature and thermal shock resistant textile yarn consisting essentially of a continuous filament fiberglass central core enclosed within a fiber cover consisting of high temperature resistant and flame retardant modacrylic fibers with at least one fiber selected from the group consisting of aramid fibers, phenolic fibers, flame retardant cellulosic fibers, polybenzimidazole fibers, partially oxidized PAN fibers and fully oxidized PAN fibers individually wrapped about the core, wherein the continuous filament fiberglass central core enclosed within the fiber cover is to form a large scale elastic cylindrical spiral around the central core which could be applied to mattress covers. Thus, the mattress covers woven from yarn have good flame-retarding effect. However, after a long time using, the fiber cover is easily worn out due to the use of fiberglass. The use of fiberglass also causes human health problems.

CN201510870804.1 discloses an expansion flame retardant system for mattress or pillow inner-sleeve filler flame retarding and a method thereof. Yarns or cloth with a raw material of cellulosic fiber or the cellulosic fiber blended with chemical fiber is processed by using an inorganic flame retardant containing nitrogen and phosphorus, so that the yarns or the cloth and the inorganic flame retardant containing the nitrogen and the phosphorus form the expansion flame retardant system which has a covering expansion function achieved when the inorganic flame retardant containing the nitrogen and the phosphorus is heated, therefore the yarns or the cloth has flame retardance. In this case, the yarns or cloth in blends are soaked in aqueous solution of inorganic flame retardant containing nitrogen and phosphorus and are dehydrated and dried. The use of glass fibre may be avoided, but once the yarns are soaked in aqueous solution of inorganic flame retardant for achieving the flame retarding effect during manufacturing process, surfaces of the yarns would be attached by the inorganic flame retardant, resulting in poor washability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a flame retardant yarn that is made by blending flame retardant viscose fiber and at least one high tenacity flame retarding fiber, and its fabrics such as mattress covers not only having good flame retardancy, but also good elasticity due to high tenacity of yarn.

To achieve the above object, the flame retardant yarn in accordance with the present invention is made by blending flame retardant viscose fiber and at least one high tenacity flame retarding fiber. The at least one high tenacity flame retarding fiber is flame retardant polyester or flame retardant finishing viscose fiber. The flame retardant viscose fiber is 40˜90% by weight of the flame retardant yarn.

The flame retardant yarn of the present invention is made of flame retardant viscose fiber and high tenacity flame retarding fiber through a ring spinning process. The ring spinning process belongs to mechanical spinning. For being twisted, spindles, rings and travelers are in use and for drafting, rollers are operated. Roving yarns are attenuated in form of silvers and the silvers are further twisted in the rings and travelers. By virtue of the bobbin winding speed faster than operating speed of the travelers, the twist of roving yarns is carried out. The travelers are driven by the bobbin through the silvers and rotating around the rings for the twist, meanwhile, rotating speed of the rings is slightly less than rotating speed of the bobbin by influence of the friction of the rings so that the rings are wound and the spinning speed is pretty high. The spinning yarn produced in the ring spinning process has a feature on winding manner that the fiber of the yarn is wound in both inward and outward directions as a conical helix shape so that the yarn is hard-packed and has high breaking strength, applicable to manufacture of high tenacity flame retardant yarn.

Ordinary viscose fiber material is of natural cellulose having abundant sources and is biodegradable, cheap, having mature processing technique and excellent mechanical properties and besides, it is not melting and dropping during combustion. However, viscose fiber is of regenerated cellulose fiber and its structure is carbohydrate with limiting oxygen index (LOI) of 19%. It is extremely flammable and has decomposition temperature of 270° C.˜350° C. and combustion temperature of 320° C.˜350° C. The ordinary viscose fiber fails to meet the flame retardancy standard of textiles. The ordinary viscose fiber thus needs to be modified by blending with flame retardant, graft copolymerization and finishing process, and turns out to be the flame retardant viscose fiber that meets the flame retardancy standard. The flame retardant viscose fiber has properties of good hygroscopicity and permeability, light texture, smooth touching feel, static resistance, bright color after dyeing or printing, etc. The flame retardant viscose fiber has the limiting oxygen index no less than 32 and is capable of being blended with cotton and wool. When the flame retardant viscose fiber is blended with other fibers, some complementary properties show up on the textile, such as physical and mechanical properties in moisture absorbing, sweat releasing, soft touching and fatigue resistance, etc.

However, the mixture of the viscose fiber and the flame retardant may have some instability. The flame retardant plays a role of electrolyte which causes aging of the viscose fiber, increase of amorphous region and decrease of crystallinity. With the addition of the flame retardant, a size of the flame retardant particles increases and more and more microgel particles are produced, thus blockage of spinning nozzles occurs. Compared with the ordinary viscose fiber, the flame retardant viscose fiber has worse spinnability and low tenacity. The tenacity of the flame retardant viscose is even down to half of tenacity of the ordinary viscose fiber. The flame retardant viscose fiber has tenacity or breaking strength of around 2.5 cN/dtex.

Polyester fiber has the advantages of high modulus, high breaking strength, high elasticity, shape preservation and heat resistance, etc., but has only around 20% in LOI, belonging to combustible fiber. In a fire accident, there is a considerable proportion that the fire is caused by fiber textiles and the molten droplet of the fiber textiles is very serious, caused by polyester fiber. Since the polyester is composed of phenyl rings belonging to macromolecular and symmetric structure, rigid phenyl rings and flexible lipid groups form conjugate structure without branches so the linear feature is great. In addition, the macromolecular structure is planar and there is Van der Waals force among molecules for molecular conjugate. For polyester macromolecule, there are two hydroxyls, one on each side, without other hydrophilic group so that the polyester has the defect of poor hygroscopicity, e.g. only 0.2% under standard conditions. However, because of high rigidity and melting point, the polyester could reach more than 40% of LOI by means of flame retardant finishing. When washing times are more than 50, LOI is unaffected and continues its upward trend. Properties such as hand handle, appearance and strength are also unaffected. One drawback is that the polyester macromolecule branches are readily degraded into flammable melting products. The molten droplet may spread the fire to other places causing a larger fire. Moreover, polyester may also cause skin burn. In spite of high tenacity, the flame retardant polyester is not suitable to be taken alone as flame retardant textile fabric because it easily melts at high temperature.

Another object of the present invention is to provide a flame retardant yarn which is made by blending flame retardant viscose fiber and the flame retardant polyester fiber, wherein the content of the flame retardant viscose fiber is more than the content of the flame retardant polyester fiber so as to make great use of both the flame retardancy of the flame retardant viscose fiber and the tenacity of the flame retardant polyester fiber.

Another object of the present invention is to provide a high tenacity flame retardant yarn which keeps excellent mechanical properties of fiber and improves the LOI of ordinary viscose fiber during flame retardant finishing. The high tenacity flame retardant yarn is obtained by blending flame retardant finishing viscose fiber and market purchase flame retardant viscose fiber with LOI no less than 32. After the last washing step of the flame retardant finishing of the ordinary viscose fiber, cellulose tows are soaked in a solution including a flame retardant for 1˜5 hours and then taken out of the solution to be dried out to obtain flame retardant finishing viscose fibers. Not only the LOI reaches above 25, the fiber strength is almost unaffected.

Accordingly, the flame retardant yarn of the present invention is made by blending the flame retardant viscose fiber and the flame retardant polyester. The weight percentages of the flame retardant viscose fiber and the flame retardant polyester are 65˜90% and 10˜35% respectively. Fabric made from the flame retardant yarn meets the flame retardancy standard of 16 CFR1633 and has good elasticity.

Preferably, the flame retardant yarn of the present invention is made by blending the flame retardant viscose fiber and the flame retardant polyester. The weight percentages of the flame retardant viscose fiber and the flame retardant polyester are 80˜90% and 10˜20% respectively. Fabric made from the flame retardant yarn meets or exceeds the flame retardancy standard of 16 CFR1633 and has a certain elasticity.

Accordingly, the flame retardant yarn of the present invention is made by blending the flame retardant viscose fiber and flame retardant finishing viscose fiber. The weight percentages of the flame retardant viscose fiber and flame retardant finishing viscose fiber are 60˜80% and 20˜40% respectively.

Accordingly, the flame retardant yarn of the present invention is made by blending the flame retardant viscose fiber, the flame retardant polyester and flame retardant finishing viscose fiber. The weight percentages of the flame retardant viscose fiber, the flame retardant polyester and flame retardant finishing viscose fiber are 40˜60%, 10˜20% and 30˜50% respectively.

Accordingly, the flame retardant yarn of the present invention contains the flame retardant viscose fiber no less than 50% by weight and having limiting oxygen index no less than 32. For the flame retardant yarn of the present invention, the flame retardant viscose fiber is used for improving the flame retardancy, so that when the flame retardant viscose fiber is blended with other fibers, LOI is still high for meeting the flame retardancy standard.

Accordingly, the flame retardant yarn of the present invention contains the flame retardant polyester no more than 30% by weight and having breaking strength no less than 5.1 cN/dtex. For the flame retardant yarn of the present invention, the flame retardant polyester is used for improving tenacity of the yarns to enhance the spinnability. Therefore, a requirement for the breaking strength of the flame retardant polyester is no less than 5.1 cN/dtex.

A method for preparing flame retardant finishing viscose fiber of the present invention is as follows.

Step 1: turning viscose fibers to cellulose filaments in a manufacture of viscose fibers, making cellulose tows by entering the cellulose filaments on a spinning machine into an acid coagulation bath, and washing the cellulose tows 2˜3 times by clean water.

Step 2: soaking the cellulose tows of step 1 in a solution dissolved with flame retardant by a bath ratio of 1˜2:10, and stirring every 30 minutes, wherein the flame retardant is 10˜15% by weight of the cellulose tows, the soaking temperature is 50˜60° C., the soaking time is 1˜5 hours.

Step 3: taking the cellulose tows out of the solution and drying the cellulose tows at a temperature below 180° C. to obtain the flame retardant finishing viscose fibers.

During process of producing the flame retardant viscose fiber, short fiber in pulp is processed and turns out to be finished fiber. The process includes four stages including viscose preparation, preparation for spinning, fiber formation, and fiber post process. The processing of the flame retardant finishing viscose fiber of the present invention belongs to the fiber post process stage. Because the viscose fiber is already formed, it affects fiber strength less. The flame retardancy is obviously improved after flame retardant finishing.

Accordingly, the flame retardant of the present invention is of phosphorus-nitrogen type.

Preferably, the flame retardant used for the flame retardant finishing viscose fiber includes phosphoric acid tris(2-chloroethyl) ether, phosphoric acid tris(2,3-dichloropropyl) ether), triphosphate(2,3-dibromopropyl) ether, dimethyl methylphosphonate ether, phosphoric acid tris (ethyl butyl) ether, or combination thereof.

Being a green and environmental protection flame retardant, no secondary pollution and no side effect on human body are characters of the flame retardant of the present invention. Ordinary viscose fiber soaked in liquid with flame retardant could inhibit combustion of viscose fiber and has great dispersibility, great coexistence with viscose fiber and has no flocculation effect. Besides, the ordinary viscose fiber does not much affect most of viscose fiber performances and is found easy in spinning.

The beneficial effects of the present invention are described below.

First, the flame retardant yarn of the present invention is made by blending flame retardant viscose fiber with superior flame retardance and high tenacity flame retarding fiber, having limiting oxygen index (LOI) no less than 32. However, breaking strength of the flame retardant viscose fiber is around 2.5 cN/dtex which is low and means bad spinnability. The present invention takes advantage of the superior flame retardance of the flame retardant viscose fiber, and takes the flame retardant viscose fiber as a major component to blend with other good tenacity and spinnability fiber such as flame retardant polyester with breaking strength no less than 5.1 cN/dtex. To ensure the requirements for flame retardancy standard are satisfied, the tenacity or breaking strength is enhanced as efficiently as possible through the adjustment of the raw materials proportioning. The flame retardant viscose fiber is 40˜90% by weight of the flame retardant yarn, conforming to the requirements of flame retardancy and tenacity. The higher the weight percentage of the flame retardant viscose fiber is, the higher the LOI or the better the flame-retarding effect is. When the weight percentage of the flame retardant viscose fiber reaches 90% and blended with other yarns, the LOI is 32 and the breaking strength is 2.1 cN/dtex. The higher the weight percentage of the flame retardant polyester is, the better the tenacity of the yarns is. When the weight percentage of the flame retardant polyester reaches 35% and blended with the flame retardant viscose fiber, the breaking strength is 3.1 cN/dtex and the LOI is 27, which performs well in the flame-retarding effect. The present invention takes advantages and avoids disadvantages of raw materials by reasonable balance to optimize flame retardancy and tenacity to produce the flame retardant yarn of high flame retardancy, long lasting flame retardancy effect, strength and elongation performance and good abrasion resistance. Fabric made by the flame retardant yarn not only has good flame retardancy, but also has good elasticity due to high tenacity of the flame retardant yarn.

Second, the process of producing the flame retardant yarn of the present invention is simple, capable of reducing cost of manufacture and raising economic benefit.

DESCRIPTION OF THE INVENTION

The embodiments of the present invention are described below for the invention easier to be understood. However, the invention should not be construed as being limited to the methods, conditions or references in the embodiments set forth herein. In contrast, the terms are provided in embodiments so that the scope of the invention will be fully understood by those skilled in the art and should not limit the scope of the invention.

Viscose fibers use natural cellulose fibers as raw materials. The raw materials are processed by alkali treatment to break hydrogen bonds inside individual molecule or between molecules, and reacted with CS₂ to form cellulose xanthate which is then by mixing, filtering, deaeration and curing etc. to obtain spinning viscose with good spinnability. A spinning process includes preparing viscose fibers that are made up of viscose in an acid bath condition by use of spinning machine, then proceeding washing, desulfurizing, bleaching, pickling, drying, etc. to obtain good quality viscose fibers. Flame retardant viscose fibers are made by making flame retardants blended in or bonded to cellulose before the normal viscose fiber spinning procedure and then proceeding the spinning process, or making the flame retardants clung to viscose fibers through physical or chemical means after the spinning process. At present, blending flame retardant technology is mainly applied in the industrial production. A dosage of the flame retardants is about 20%. However, poor compatibility between flame retardants and cellulose leads to some deterioration of viscosity of spinning solution and mechanical property of fibrous materials. The present invention provides flame retardant viscose fibers that have a limiting oxygen index no less than 32.

A process in ring spinning of the flame retardant yarn of the present invention has following steps:

A) making composite fibers by mixing or blending flame retardant viscose fibers and high tenacity flame retarding fibers in certain weight ratio.

B) proceeding a blowing process which includes opening the composite fibers of step A by an opening and picking machine to obtain fibrous rolls by net amount of 350 g/m and length of 25 m.

C) making card silvers in the net amount of 20 g/5 m by carding the fibrous rolls of step B in use of a carding machine.

D) making drawing silvers in the net amount of 16 g/10 m by combining three flame retardant viscose fibers and one high tenacity flame retarding pre-drawing fiber by a drawing frame.

E) making roving yarns in the net amount of 4.0 g/10 m and twist of 3.5/10 cm by stretching the drawing silvers of step D by a roving frame.

F) proceeding a spinning process, which includes spinning the roving yarns of step E by a compact spinning device to obtain blended compact yarns in the net amount of 1.6 g/100 m and twist of 80/10 cm.

G) proceeding a heat setting twist process, which includes twisting the blended compact yarns of step F to eliminate internal stress of yarns and stabilize the twist.

H) proceeding a spooling process, which includes winding the twisted blended compact yarns of step G on a bobbin to obtain yarns.

A method for preparing the flame retardant finishing viscose fiber is as follows:

(1) turning viscose fibers to cellulose filaments in a manufacture of viscose fibers, making cellulose tows by entering the cellulose filaments on spinning machine into an acid coagulation bath, and washing the cellulose tows 2˜3 times by clean water;

(2) soaking the cellulose tows of step (1) in a solution including a flame retardant such as phosphoric acid tris(2-ethylhexyl) ether with a bath ratio of 1:10, and stirring every 30 minutes; wherein the flame retardant is 10% by weight of the cellulose tows, the soaking temperature is 50˜60° C., and the soaking time is 5 hours;

(3) taking the cellulose tows out of the solution and drying them at a temperature below 180° C. to obtain flame retardant finishing viscose fibers.

In the above method, the flame retardant may be phosphoric acid tris(2-chloroethyl) ether, phosphoric acid tris(2,3-dichloropropyl) ether), triphosphate(2,3-dibromopropyl) ether, dimethyl methylphosphonate ether, phosphoric acid tris (ethyl butyl) ether, or combination thereof.

A flame retardancy test for the flame retardant yarn and the fabrics made from the flame retardant yarn:

Limiting oxygen index (LOI) expresses the minimum concentration of oxygen in a mixture of oxygen and nitrogen as a percentage of volume. In a condition of ASTMG/D2863, materials keep burning under an initial room temperature.

Combustion residual rate is obtained by calculating material mass before and after combustion for percentage of residual mass. The larger the combustion residual rate is, the less the amount of flammability molecules produced by thermolysis of the material and the better the flame retardancy is.

For mattress woven from the flame retardant yarn of the present invention, the combustion test is based on the flame retardancy standard of 16 CFR1633. An injection flame burner is used in the combustion test of the surface of the mattress. Evaluation indices includes: (1) thermal release rate peak is less than or equal to 200 kW in a 30-minute test; (2) thermal release amount is less than or equal to 5 MJ in the first 10 minutes; (3) out of at least three samples tested, if any of the three samples fails to meet above both requirements, the product is unqualified.

Example 1

Based on a total weight of the flame retardant yarn, weigh for 90 wt % of flame retardant viscose fiber and 10 wt % of flame retardant polyester, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 16 yarn-count knitting yarn by a ring spinning frame.

Example 2

Based on a total weight of the flame retardant yarn, weigh for 80 wt % of flame retardant viscose fiber and 20 wt % of flame retardant polyester, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 21 yarn-count knitting yarn by a ring spinning frame.

Example 3

Based on a total weight of the flame retardant yarn, weigh for 75 wt % of flame retardant viscose fiber and 25 wt % of flame retardant polyester, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 26 yarn-count knitting yarn by a ring spinning frame.

Example 4

Based on a total weight of the flame retardant yarn, weigh for 65 wt % of flame retardant viscose fiber and 35 wt % of flame retardant polyester, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 32 yarn-count knitting yarn by a ring spinning frame.

Example 5

Based on a total weight of the flame retardant yarn, weigh for 80 wt % of flame retardant viscose fiber and 20 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 16 yarn-count knitting yarn by a ring spinning frame.

Example 6

Based on a total weight of the flame retardant yarn, weigh for 75 wt % of flame retardant viscose fiber and 25 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 21 yarn-count knitting yarn by a ring spinning frame.

Example 7

Based on a total weight of the flame retardant yarn, weigh for 70 wt % of flame retardant viscose fiber and 30 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 26 yarn-count knitting yarn by a ring spinning frame.

Example 8

Based on a total weight of the flame retardant yarn, weigh for 60 wt % of flame retardant viscose fiber and 40 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 32 yarn-count knitting yarn by a ring spinning frame.

Example 9

Based on a total weight of the flame retardant yarn, weigh for 40 wt % of flame retardant viscose fiber, 10 wt % of flame retardant polyester and 50 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 16 yarn-count knitting yarn by a ring spinning frame.

Example 10

Based on a total weight of the flame retardant yarn, weigh for 50 wt % of flame retardant viscose fiber, 10 wt % of flame retardant polyester and 40 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 21 yarn-count knitting yarn by a ring spinning frame.

Example 11

Based on a total weight of the flame retardant yarn, weigh for 55 wt % of flame retardant viscose fiber, 10 wt % of flame retardant polyester and 35 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 26 yarn-count knitting yarn by a ring spinning frame.

Example 12

Based on a total weight of the flame retardant yarn, weigh for 60 wt % of flame retardant viscose fiber, 10 wt % of flame retardant polyester and 30 wt % of flame retardant finishing viscose fiber, take them into a regular cotton spinning equipment to prepare cotton blended silver. The cotton blended silver is eventually spun into 32 yarn-count knitting yarn by a ring spinning frame.

The yarns of the above examples 1˜12 are made into cloth and proceeded with performance test. When they are all satisfied with the flame retardancy standard of 16 CFR1633, following performances are tested:

TABLE 1 performance test tensile breaking Combustion yarn weight, breaking strength, residual count, s g/m² force, N cN/dtex capacity, % LOI Example 1 16 320 1250 2.1 27 32 Example 2 21 265 1546 2.3 24 30 Example 3 26 220 2387 2.7 21 28 Example 4 32 180 2530 3.1 18 27 Example 5 16 320 1198 2.0 26 31 Example 6 21 265 1439 2.0 23 30 Example 7 26 220 2156 2.5 19 28 Example 8 32 180 2398 2.9 17 27 Example 9 16 320 1367 2.0 25 29 Example 10 21 265 1689 2.2 23 28 Example 11 26 220 2487 2.7 21 26 Example 12 32 180 2658 2.9 16 27

According to the Examples 1˜12, it is concluded that when the LOI is larger than 26, the flame retardancy standard of 16 CFR1633 could be achieved. The fabric has breaking strength larger than 2.0 cN/dtex as better performance in tenacity. The higher the weight percentage of the flame retardant viscose fiber is, the higher the LOI or the better the flame-retarding effect is. When the weight percentage of the flame retardant viscose fiber reaches 90% and blended with other yarns, the LOI is 32 and the breaking strength is 2.1 cN/dtex. The higher the weight percentage of the flame retardant polyester is, the better the tenacity of the yarns is. When the weight percentage of the flame retardant polyester reaches 35% and blended with the flame retardant viscose fiber, the breaking strength is 3.1 cN/dtex and the LOI is 27, which performs well in the flame-retarding effect. 

What is claimed is:
 1. A flame retardant yarn, wherein the flame retardant yarn is made by blending flame retardant viscose fiber and at least one high tenacity flame retarding fiber, the at least one high tenacity flame retarding fiber is flame retardant polyester or flame retardant finishing viscose fiber; wherein the flame retardant viscose fiber is 40˜90% by weight of the flame retardant yarn.
 2. The flame retardant yarn as claimed in claim 1, wherein the flame retardant yarn is made by blending the flame retardant viscose fiber and the flame retardant polyester, the weight percentages of the flame retardant viscose fiber and the flame retardant polyester are 65˜90% and 10˜35% respectively.
 3. The flame retardant yarn as claimed in claim 1, wherein the flame retardant yarn is made by blending the flame retardant viscose fiber, the flame retardant polyester and flame retardant finishing viscose fiber, the weight percentages of the flame retardant viscose fiber, the flame retardant polyester and flame retardant finishing viscose fiber are 40˜60%, 10˜20% and 30˜50% respectively.
 4. The flame retardant yarn as claimed in claim 1, wherein the flame retardant yarn is made by blending the flame retardant viscose fiber and flame retardant finishing viscose fiber, the weight percentages of the flame retardant viscose fiber and flame retardant finishing viscose fiber are 60˜80% and 20˜40% respectively.
 5. The flame retardant yarn as claimed in claim 1, wherein the flame retardant yarn comprises the flame retardant polyester no more than 30% by weight and having breaking strength no less than 5.1 cN/dtex.
 6. The flame retardant yarn as claimed in claim 2, wherein the flame retardant yarn comprises the flame retardant polyester no more than 30% by weight and having breaking strength no less than 5.1 cN/dtex.
 7. The flame retardant yarn as claimed in claim 3, wherein the flame retardant yarn comprises the flame retardant polyester no more than 30% by weight and having breaking strength no less than 5.1 cN/dtex.
 8. The flame retardant yarn as claimed in claim 1, wherein the flame retardant yarn comprises the flame retardant viscose fiber no less than 50% by weight and having limiting oxygen index no less than
 32. 9. The flame retardant yarn as claimed in claim 2, wherein the flame retardant yarn comprises the flame retardant viscose fiber no less than 50% by weight and having limiting oxygen index no less than
 32. 10. The flame retardant yarn as claimed in claim 3, wherein the flame retardant yarn comprises the flame retardant viscose fiber no less than 50% by weight and having limiting oxygen index no less than
 32. 11. The flame retardant yarn as claimed in claim 4, wherein the flame retardant yarn comprises the flame retardant viscose fiber no less than 50% by weight and having limiting oxygen index no less than
 32. 12. A method for preparing flame retardant finishing viscose fiber, wherein the method includes: step 1: turning viscose fibers to cellulose filaments in a manufacture of viscose fibers, making cellulose tows by entering the cellulose filaments on a spinning machine into an acid coagulation bath, and washing the cellulose tows 2˜3 times by clean water; step 2: soaking the cellulose tows of step 1 in a solution dissolved with flame retardant by a bath ratio of 1˜2:10, and stirring every 30 minutes, wherein the flame retardant is 10˜15% by weight of the cellulose tows, the soaking temperature is 50˜60° C., the soaking time is 1˜5 hours; step 3: taking the cellulose tows out of the solution and drying the cellulose tows at a temperature below 180° C. to obtain the flame retardant finishing viscose fibers.
 13. The method as claimed in claim 12, wherein the flame retardant is of phosphorus-nitrogen type including phosphoric acid tris(2-chloroethyl) ether, phosphoric acid tris(2,3-dichloropropyl) ether), triphosphate(2,3-dibromopropyl) ether, dimethyl methylphosphonate ether, phosphoric acid tris (ethyl butyl) ether, or combination thereof.
 14. A high tenacity flame retarding fabric, made by the flame retardant yarn as claimed in claim
 1. 15. The high tenacity flame retarding fabric as claimed in claim 14, wherein the flame retardant finishing viscose fiber is prepared by a method includes: step 1: turning viscose fibers to cellulose filaments in a manufacture of viscose fibers, making cellulose tows by entering the cellulose filaments on spinning machine into an acid coagulation bath, and washing the cellulose tows 2˜3 times by clean water; step 2: soaking the cellulose tows of step 1 in a solution including a flame retardant with a bath ratio of 1˜2:10, and stirring every 30 minutes, the flame retardant is 10˜15% by weight of the cellulose tows, the soaking temperature is 50˜60° C., the soaking time is 1˜5 hours; step 3: taking the cellulose tows out of the solution and drying the cellulose tows at a temperature below 180° C. to obtain flame retardant finishing viscose fibers. 